Textile Yarn Dyeing - Tips & Techniques

Cotton yarn dyeing techniques explained in detail right from manual dyeing to latest automated machine dyeing. Tips on bleaching, dyeing and finishing along with many frequently faced problems are addressed in this blog.

May 26, 2009

Textile fibers are subjected to various physical operations to make in to a yarn. For example cotton fiber passes through opening, carding, drawing and spinning to become a yarn. During these phases the original moisture content on the fiber would have been lost and some static electricity would be carried by the fiber. The amount of static current carried by yarn changes from fiber to fiber. Similarly the strength of any fiber depends up on how close the present moisture content is to the original natural value.

Similarly some high twist yarn would tend to loose its twist as and when it is allowed freely, making a lengthwise elongation.

Some fibers would tend to shrink when exposed to hot atmosphere or any treatment that involves heat and hence higher temperature. For example polyamide, polyester etc.

Some blends like Cotton/Lycra, Viscose/Lycra require conditioning to make the width the fabric stable.

So all the above said factors, if not addressed properly would reflect badly on the final quality of yarn or fabric.

A process that addresses all the above parameters is called CONDITIONING. Conditioning process differs from fiber to fiber.So conditioning is a preliminary process in any processing that improves and maintains the quality of yarn.

Conditioning may be done in yarn stage on perforated paper or plastic cones/cheeses in an Auto Clave or Horizontal beam dyeing machine as shown above.

Conditioning Process for various yarns:

1) 100% Polyester yarn: Load the yarn in the form cones wound on plastic cones, in to a beam dyeing machine. Introduce steam and raise the temperature to 100°C at 3°C per minutes. Steam for 15 minutes at 100°C followed by 15 minutes cooling = 1 cycle. Repeat the cycle for 4 times.

2) 100% Nylon: Load the yarn in the form cones wound on plastic cones, in to a beam dyeing machine. Introduce steam and raise the temperature to 100°C at 3°C per minutes. Steam for 15 minutes at 100°C followed by 15 minutes cooling to a temperature of 50°C = 1 cycle. Repeat the cycle for 4 times.

3) Silk yarn: Load the yarn in the form cones wound on plastic cones, in to a beam dyeing machine. Introduce steam and raise the temperature to 70°C at 3°C per minutes. Steam for 15 minutes at 70°C followed by 15 minutes cooling to a temperature for 30°C = 1 cycle. Repeat the cycle for 4 times.

3. If you want to use regular cheese weights of 800 to 1000 grams, then the cheeses have to be conditioned in the autoclave with moist steam at 100°C for 30 minutes, repeatedly, so that a stable shrinkage percentage of yarn is reached. The linear shrinkage % should be 20 to 25%.

LYCRA® is a man-made elastic fibre invented and produced only by DuPont®.It’s remarkable properties of stretch and recovery enhance all fabrics and garments in which it is used, adding easy comfort and freedom of movement and improving fit and shape retention. Swimwear and lingerie owe their figure-flattering fit to LYCRA®. All types of hosiery are softer, smoother-fitting and more durable thanks to LYCRA®. In short, a little bit of LYCRA® makes all types of apparel fit better, feel better and look better. Tech-Talk

LYCRA® belongs to the generic elastane classification of man-made fibres(known as spandex in the US and Canada) and is described in technical terms as a segmented polyurethane it is composed of “soft”, or flexible, segments bonded together with “hard”, or rigid, segments. This gives the fibre it’s built-in, lasting elasticity.

LYCRA can be stretched four to seven times its initial length, yet springs back to it’s original length once tension is released.

While LYCRA® appears to be a single continuous thread, it is in reality a bundle of tiny filaments.

How LYCRA® is used

LYCRA® is never used alone; it is always combined with another fiber (or fibers), natural or man-made. Fabrics enhanced with LYCRA® retain the appearance of the majority fibre.

The type of fabric and it’s end use determine the amount and type of LYCRA® required to ensure optimum performance and aesthetics. As little as 2 percent LYCRA® is enough to improve a fabric’s movement, drape and shape retention, while fabrics for high-performance garments such as swimwear and active sportwear may contain as much as 20-30 percent LYCRA®. Weaving or knitting techniques, togheter with fabric type and end use, determine whether LYCRA® is used in a bare or covered yarn form.

Single and double covered LYCRA®

Core-twisted LYCRA®

The material used in the making of the Cotton-Lycra line of Snob underwear consist of:

October 20, 2008

• Like polyester fiber, (polyamide) nylon also requires to be heat set prior to any wet processing. Nylon can be best heat set in aqueous water bath at about 120°C for half an hour with a dispersing agent like Ekaline FI. This treatment should be followed by hot and cold washes. Then dry the yarn and rewind it on parallel winding machine.

• Pretreatment & Dyeing:

1. Sodium Acetate = 1% 2. Acetic acid = 1% 3. Glauber salt = 10 % Run at 50°C for one cycle. Now add the dissolved dyestuff at the same temperature. Run at 75° for 10 minutes and raise the temp to 95°C and continue dyeing; add a leveling agent such as Lyogen SE at this temp (for dark shades 1% and for light shades up to 4% or as recommended by the manufacturer).

• After treatment:

1. After thorough hot and cold washes, a fixing treatment is required for dark and medium shades. There are products like Lyogen PA liquid are available for this purpose and the user should follow the instructions of the manufacturer. 2. Finishing of nylon yarn can be made using some anti-static agent in the final bath. 3. Hydro extract, dry and rewind.

May 06, 2008

In the previous post we have discussed the chemical aspects only and the processes of reaction with cellulose and the water have been treated as if they were occurring quite separately. But in fact this is not the case and we have just seen the importance of exhaustion in obtaining a good efficiency.

We will now examine what happened in a two stage dyeing process in which the dyestuff is exhausted from the neutral dye bath in the first stage and then at a later stage the solution is made alkaline so that the reaction begins.

In the first stage of neutral dyeing, no decomposition of dye takes place and the process is exactly the same as the dyeing of a direct dye. The only difference is the lower degree of exhaustion of reactive dyes. At the end of the 1st stage we have two equilibrium.

When alkali is added to the system, chemical reaction begins. In the dye bath hydrolysis with water occurs, while in the fibre, the dissolved dye will also hydrolyse, but the adsorbed dye will mainly react with the fibre although the possibility of aqueous hydrolysis cannon be excluded. The hydrolysed dye (DOH) will have similar properties to the parent dye ( or direct dye) and will not get adsorbed on the fibre surface. Finally, when all the reactive dye present has been destroyed one way or another, new equilibrium will be set up between the hydrolysed dye in the dye bath, in solution inside the fiber and adsorbed on cellulose while the combined dye is present as a separate component, not taking part in this.

April 27, 2008

The rate of reaction with cellulose may be written:Rate = Kc (D.Cl)f (Cell O)The rate constants of reactive dye with cellulose may not be available but the rate constant with water is known and since the two are proportional, may be used as a guide to behavior. The rate constants of some procion dyes are shown in the table below. The rate of reaction of Procion “H” at 20°C is extremely low ad difficult to measure. The values shown in the table are extrapolation from the values at 50°C and 70°C.

Dyes

Bi molecular reaction

Constant 20°C

1 gram mloe/c- 50°C

1 gram mloe/c- 50°C

Orange 2R

13.5

Red 8B

13.2

Yellow GR

11.0

Blue 3G

3.22

Yellow 6G

2.82

Scarlet G

2.06

Orange G

1.61

Rubine HB

0.18

1.99

9.99

Yellow H3B

0.054

0.81

5.06

Yellow H3G

0.034

0.55

3.62

Blue HB

0.044

0.46

2.23

Blue HGR

0.027

0.34

1.89

It will be observed that there are significant differences between the reactivity of individual dyes in each group the most reactive being roughly 10 times more reactive than the least reactive. However the difference between hot brand and cold brand is even more marked, the latter being 50 times as active as the former.

The rate constant of a chemical reaction increases with increasing temperature by between two and three times for every 10↑8C increase in temperature. Clearly then an increase in temperature of 50° (from 20 to 70°) may be expected to increase the rate 50 times. This is seen in the above table, where rate constants pf Procion H dyes at 70°C are similar to those of Procion dyes at 20°C.

Rate of reaction can be changed by varying the concentration of cellulose ions in the fibre, by changing the pH of the external bath.

If 1 unit pH of the dye bath is increased, the concentration of cellulose ions will increase tenfold. An increase of 1.7 pH units will increase the concentration 50 fold and the rate of reaction similarly. Thus Procion H dyes at pH 12.5 should react at the same rate as the Procion M dyes at pH 10.5. This proves to the case but the yield of the combined dye is relatively low.

The reason is, if the pH exceeds 12, the exhaustion of dye bath falls rapidly. Below pH 11, the concentration of cellulose ions is small compared with that of dye, at pH 11 it is roughly equal and at pH 12 it is considerably greater than that of the dye. Because of the increasing ionization the fibre acquires a large negative charge that depresses the absorption of the dye.

Thus the degree of exhaustion at pH 12 is so low that though the reaction takes place with cellulose in cold in one hour, the efficiency is low.

April 24, 2008

What are Fibre-Reactive Dyes?DefinitionA fibre-reactive dye will form a covalent bond with the appropriate textile functionality. This is of great interest, since, once attached, they are very difficult to remove.Early fibre-reactive dyesThe first fibre-reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially.Although fibre-reactive dyes have been a goal for quite some time, the breakthrough came fairly late, in 1954. Prior to then, attempts to react the dye and fibres involved harsh conditions that often resulted in degradation of the textile.The first fibre-reactive dyes contained the 1,3-5-triazinyl group, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fibre degradation occurred. ICI launched a range of dyes based on this chemistry, called the Procion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colours. Procion dyes could also be applied in batches, or continuously.The general structure of a fibre-reactive dye is shown below:

Note the four different components of the dyeThe chromogen is as mentioned before (azo, carbonyl or phthalocyanine class).The water solubilising group (ionic groups, often sulphonate salts), which has the expected effect of improving the solubility, since reactive dyes must be in solution for application to fibres. This means that reactive dyes are not unlike acid dyes in nature.The bridging group links the chromogen and the fibre-reactive group. Frequently the bridging group is an amino, -NH-, group. This is usually for convenience rather than for any specific purpose.The fibre-reactive group is the only part of the molecule able to react with the fibre. The different types of fibre-reactive group will be discussed below.A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilise as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O- groups. These can then attack electron-poor regions of the fibre-reactive group, and perform either aromatic nucleophilic substitution to aromatics or nucleophilic addition to alkenes.Nucleophilic substitutionAromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favorable p-electron system. However, nucleophilic substitutions are not very common on aromatics, given their already high electron density. To encourage nucleophilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack.

For example:

But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added.

However, this will only work if there is a good leaving group, such as -Cl or -N2.The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye2: (This is the same as the chime molecule at the top of the page)

Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilized as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens:

One problem is that instead of reacting with the -OH groups on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolyzed. The two reactions compete, and this unfavourable because the hydrolyzed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water.Nucleophilic additionAlkenes are quite reactive due to the electron-rich p-bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favored generally, because of the repulsion between the Nu- and the electron-rich p-bond. However, they will occur if there are sufficient electron withdrawing groups are attached to the alkene, much as before, with aromatic substitution. In this case, the process is known as Michael addition or Conjugate addition.For this reaction type, the most important dye class is the Remazol reactive dye. This dye type reacts in the presence of a base such as HO-. The mechanism for the reaction of one of these dyes is shown below:As before, the intermediate is resonance stabilized, but this has not been shown.

April 20, 2008

Introduction:Reactive dyes are probably the most popular class of dyes to produce 'fast dyestings' on piece goods. These were first introduced a little over 40 years based on a principle which has not been used before. These dyes react with fibre forming a direct chemical linkage which os not easily broken.Their low cost, ease of application, the bright shades produced by them coupled with good wash fastness make them very popular with piece good dyers. Even in thereads these classes are gaining in popularity for cotton sewings.

Chemistry of reactive dyes:Reactive dyes differ from other colouring matters in that they enter in to chemical reaction with the fibre during the dyeing process and so become a part of the fibre substances.A reactive dye may be represented asR - B - Xwhere R - ChromogenB - Bridging groupX - Reactive systemWhen this reacts with the fibre, F, it formsR - B - X - FThe wet fastness of the dyesings produced depends on the stability of the true covalent bond X-F.

Reactive Systems:Some of the popupar reactive systems is use today:Reactive dyes are based on Cyanuryl chloride. The cold brand dyes (M brand) are based on dichloro triazinyl derivatives whereas the "H" brands are monochloro triazinyle derivatives.The reactivity of the chlorine atoms decreases greatly as they are successively substituted. Thus the dichloride derivative (M) is more reactive than the mono chloro reactive (H) dyes. This is shown by the fact that "M" dyes will react readily with cellulose at room temperatire in the presence of mild alkalies such as sodium carbonate, whwere as "H" dyes need to be heated at least to 60°C and require more strongly alkalines before reaction will take place at a reasonable rate.The other popular systems are based on Vinyl suplhones (Remazols) and trichloro pyrimidyl. The Remazols are very popular for discharge printing and can given excellant white discharge from a dark base.Reference: Textile Processing Guide

In order to clarify what fastness means, we shall examine various fastness properties that a thread or yarn may be required to have. We shall study how grades of fastness are established and what they signify and finally we should take a look at what fastness you can expect from various classes of dyestuff on the major substrates of interest to us.

Fastness Properties:

The most desirable fastness properties in any thread or yarn is arguably of wash fastness.

In general, the dyers used to carry out two main wash fastness tests, viz.,

Test-1: A length of coloured thread is plaited with a white partner and treated at 95°C in an alkaline soap solution for 30 minutes. The degree of staining on to the adjacent white thread ( which can be of one or more white substrates) is assessed as in the change of shade of the original colour. The test is commonly known as ISO4.

Test-2: As in Test-1 above but treatment is only at 60°C. The test is called ISO3.

Another popular fastness demand is to rubbing both wet and dry with the sample being hand or machine rubbed. Only the staining on the white calico test fabric is recorded.

Fastness to light is either carried out in sunlight ( a low method) or in an artificial Xenon lamp tester (much faster). Along with the test sample are eight blues of knownlight fastness which fades to the same degree as the sample gives the light fastness grading of the sample, only change of the shade is recorded.

Fastness to bleach, either peroxide or hypochlorite are severe tests where normally only change of shade need be recorded.

A less severe bleach type test is fastness to chlorinated water which is meant to represent the effect of swimming pool waters on textiles; usually for swim and beach wear garments.

Fastness to hot pressing at a wide range of temperatures with both change of shade and staining being relevant. Disperse dyes on polyester can sublime ( literally evaporate) on some severe permanent pleating processes and even at low iron heats many classes of dye will stain ( but may not do so on ISO3 wash tests) white fabric on contact.

There are some exotic fastness requirements like fastness to vulcanizing , a process used to cure rubber footwear or fastness tostone washing, a fickle process used to fade cotton denim jeans.

Fastness Grades:

Nearly all fastness properties are assessed on a scale of 1 to 5 with 5 being the best rating and 1 the worst. The Grey Scale I can be used to assess the change in shade. Staining scales are slightly different but the same usage principle applies.

Exceptionally, light fastness is measured on a scale of 1 to 8 with 8 being the best and 1 the worst.

April 18, 2008

Machinery details:In its simplest form a package-dyeing machine is a vessel capable of containing packages of textile material through which heated dye liquor is passed by means of a circulation pump. Later developments accelerated by the need to dye polyester at temperatures above the boil lead to enclosing and strengthening such vessels so that they could operate upto 140° c at pressures around 70 psi (4.95 kgs per/ sq. cm.)

Accessories were added to allow samples to the extracted without depressurizing the whole system and to inject dyes and chemicals from out with the main circulation circuit. Later still, simple controls of time and temperature were replaced with fully automatic programmes based on sophisticated microprocessors that reduced operator involvement in the dyeing process to a minimum and elevated LOA (Limits Of Accuracy) sophistication previously unattainable levels.

Liquor flow: We expect a main circulating pump to deliver 30 litres of liquor per kg of thread at 1.26 kgs per sq.cm. pressure which usually means the bath is pumped through the thread load upto four to five times per minute.

Many of us, when faced with an unlevel cheese dye lot, blame our troubles on poor liquor flow which, because the dyeing process, by necessarily, is unobservable and because there is no instrument to read out the flow. Is hard to prove one way or another.

But a small one, however, can interpret the evidence available to him e,g,, here are a few tips on how to ascertain whether or not abnormal liquor flow is the source of Unlevelness.

1. Check the in-out and out-in pressure gauges and compare the readings with your past experience. Your Dalal and Staffi machines with their modest pumps should register a pressure differential of around 0.5 kg per sq.cm. If the differential is significantly lower than this value, liquor may be freewheeling or channeling through a badly seated carrier, a sprung cap or a loosely loaded column of cheeses.

2. Likewise pressure differential higher than 0.5 kgs per sq.cm could indicate that something is causing unduly high back pressure e.g. very dense cheeses.

3. Unlevelness on a number of cheeses which represent one spindle or multiples of one spindle might indicate poor sealing of the number(s) of spindles involved.

4. Unlevelness on a number of cheeses that represent one complete layer as horizontal cross section of a carrier load of cheeses may mean that the machine has (leveled out) for sampling or during a power failure exposing the top most layer of cheeses to oxidation or differential dye uptake.

5. Loss of air pad pressure in one way low liquor dyeing can cause reduced liquor flow.

Open expansion tank: This tank is sized so that the top row of cheeses is exposed when liquor is leveled bag to the expansion tank from the kier by gravity.

The tank feeds the suction side of the secondary pump, which normally discharges into the main pump housing via the non-return valve.

The expansion tank is an invaluable aid to level dyeing as it allows controlled additions of chemicals and redip dyes, when pressurized.

Extraction rate from the expansion tank is usually 5 to 25 litres per minute with the pump running at a pressure of around 3.6 kgs. per sq. cm.

It is important that the right balance between expanding main kier liquor and expansion tank injection rate is struck otherwise liquor flow may be affected. This balance is obtained by drilling out the orifice plate on the cooled liquor return from the main kier to the expansion tank.

The efficiency of the back cooler or condenser is also important since if the temperature in the expansion tank is allowed to rise about 80 to 85°C, the adversely secondary pump may cavitate thus affecting the flow characteristics of the dyeing system. If the liquor is over cooled, energy is wasted in reheating it in the main kier and of course cooling water volumes are unnecessarily high.

April 12, 2008

A number of one bath methods have been developed for cellulosic blend dyeing. Disperse/direct dyeing of cotton/nylon or polyester is well known although of little practiced use because of the poor wet fastness, except in the pale shades.

Disperse/Reactive: Methods based on hot dyeing reactive dyes in which the disperse and reactive dyes are added to the bath with 5 gram/liter of Resist salt L (m-nitro-sodium-benzene-sulphuric acid) which prevents hydrolysis of reactive group. Dyeing of polyester is first carried out at 120°C. Then the bath is cooled to 80°C. Electrolyte is added and dyeing of the cotton proceeds in the usual way.

Disperse/Vat:Disperse and vat (pigment) dyes are added to the bath plus a large quantity of dispersing agent. Dyeing at 100° to 130°C proceeds. Then the bath is cooled to 80°C and caustic soda and hydros are added and dyeing of the cotton carried out. There are several Union dyes (Cottestren) on the market using this principle. These commercial blends have to be formulated for a predetermined fibre mix and may turn out to be uneconomic for a customer's particular end use and further more may not give solid shade dyeing under adverse conditions of applications.

Nylon/Cotton:A Hoechst patent for single bath application reactive/metal complex dyes has the following method. A dyestuff and alkali to give a pH of 8 to 12 and raise temperature to 80°C to dye the cotton. The pH is then reduced to 6.5 to 7 by the addition acid. The temperature is raised to 95°C and the nylon portion is dyed. Acid dyes will precipitate under these conditions, metal complex dyes will not.

Naphthol dyestuff have traditionally been applied by the multi stage route of impregnation, hydro extraction/rinsing and development.German and Swiss manufacturers have now developed a one bath application process which offers a real rationalization of the dyeing process. This one bath process is cotton in hank in open becks and spray dyeing units and cotton piece on the winch.The basis of the method is to retain the bath after impregnation and maintain the color pigment which is formed during development in highly dispersed form, by means of a special auxiliary. The one bath method consist of

impregnation for about 20 minutes. at 20 to 30°C.

addition of acid and dizao solution without letting off the bath.

coupling the dyestuff of about 30 minutes

cleansing after treatment.

A fairly wide selection of naphthol/base combinations are suitable for this process.

April 03, 2008

Liquid ammonia can be regarded simply as a new medium for tailoring the dimensions and properties of cellulosic materials to shrink, swell, stretch and relax and can therefore be used to obtain a variety of effects on many materials.The economics of cotton yarn manufacture hinge on the price of raw material comprising it, e.g. more than 25% of the cost of a cotton sewing thread is accounted for by the raw cotton price. An accepted yardstick of a cotton is the strength it produces in yarn and thread forms and it was to this end that much of the development work of the liquid ammonia process was designed.

Properties of Liquid Ammonia Treated yarn:The following properties have been established for liquid ammonia treated yarns:

Tensile strength significantly increases.

The elongation at break is only about 2/3 that of untreated yarn.

Loop strength and knot strength increases slightly.

Abrasion resistance is reduced but this decrease is less than caustic soda mercerising.

After bleaching or dyeing, treated yarns have virtually zero shrinkage when treated in boiling water.

A pleasing lustre is imparted to the treated yarns albeit slightly less than for caustic mercerising.

Dye affinity is increased by 3/4 of the amount attained by caustic soda mercerising.

Moisture absorption is increased but again to some what lesser degree than for caustic mercerising.

The elimination of hank winding is possible, due to the high speed reaction in liquid ammonia which permits package to package processing.

Maximum strength increases, require maximum stretch in the ammonia moving zone but this is difficult to apply without breakage to yarns. However, if the stretch is reduced and more modest strength increases accepted ( of the order of 20% - 30%) is readily possible to liquid ammonia treat singles yarn. This is a sharp contrast to the difficulties in processing singles yarn by mercerising.

It is therefore possible to produce this means a lustrous singles yarn for use in weaving and knitting applications.

From ecological view point also, ammonia is more readily and cheaply recoverable than caustic soda mercerising liquors which produce effluent and which has to be disposed of. The problem of caustic liquor discharge to rivers is so acute in some countries that permission to erectmercerising plants is difficult to obtain.

Early difficulties of dye affinity variations between packages of liquor ammonia treated yarns have now been eliminated by improved control of the treatment process.

The technological difficulties of converting pressurised liquid ammonia and recovering pressurised liquid ammonia from the gas evolved during the process, have been successfully overcome.